A thermal flow meter is used under very severe environments in some cases. For example, in a case where the thermal flow meter is mounted on a vehicle to measure a flow rate of an intake air to an internal combustion engine, water together with the intake air may enters a bypass passage through which a measurement target gas of the thermal flow meter flows. In some cases, the water may remain inside the bypass passage.
The thermal flow meter measures the flow rate by performing heat transfer between the measurement target gas flowing through the bypass passage and a flow rate detection circuit. Therefore, when the water enters the bypass passage, it is difficult to accurately measure the flow rate. In addition, the flow rate detection circuit may be damaged (for example, erosion) due to the water. For this reason, there is a known technology in which a through hole penetrating the bypass passage from the inside thereof to the outside is provided in order to discharge the water in the bypass passage to the outside of the thermal flow meter, so that the water in the bypass passage is discharged to the outside of the thermal flow meter through the through hole. Such a technology is disclosed, for example, in JP 2006-162631 A.